Influence of Li concentration on structural, morphological and electrochemical properties of anatase-TiO2 nanoparticles

Lithium-doped anatase-TiO2 nanoparticles (LixTi1-xO2 NPs, x = 0, 0.05, 0.10, 0.15 and 0.20) could be synthesized by a simple sol–gel process. X-ray diffraction (XRD) results displayed the tetragonal (space group: I41/amd) of polycrystalline TiO2 anatase phase. The spectroscopy results of Raman and FT-IR confirmed the anatase phase of TiO2 through the specific modes of metal oxides vibration in the crystalline TiO2. Surfaces micrographs by scanning electron microscope (SEM) of agglomerated LixTi1-xO2 NPs showed a spongy like morphology. Transmission electron microscope (TEM) illustrated a cuboidal shape of dispersed NPs with particle size distributed in a narrow range 5–10 nm. Bruanauer Emmett-Teller (BET) results showed the increased surface area of LixTi1−xO2 NPs with increasing Li content. LixTi1-xO2 NPs (x = 0.05–0.20) working electrodes illustrated a pseudocapacitive behavior with excellent electrochemical properties through the whole cycles of GCD test. Interestingly, Li0.1Ti0.9O2 NPs electrode illustrated a high performance in terms of maximum specific capacitance 822 F g−1 at 1.5 A g−1 in 0.5 M Li2SO4 electrolyte, with excellent capacitive retention 92.6% after 5000 cycles GCD test.


Synthesis of pure and Li-doped anatase TiO 2 NPs
In the process of synthesis pure anatase TiO 2 NPs, 5 ml polyethylene glycol was added to a well stirred deionized (DI) water:ethanol solution with 4:1 ratio by volume (40:10 ml).Then 10 ml of C 12 H 28 O 4 Ti was gradually dropped to this solution, while vigorously stirred at room temperature on a magnetic stirrer hot plate for further 20 min.Then, 2.5 wt.% aqueous ammonia (NH 4 OH) was added dropwise to carefully controlled the pH at 7, and further stirring for 30 min.After that, increased the temperature of a solution to 60 °C and kept on stirring until a wet gel was formed, and allowed to dry at 75 °C.The final product was achieved by crushing the dried gel, ground to fine powder and pyrolized at 500 °C in a furnace for 2 h, using a heating rate of 2 °C/min.Li x Ti 1-x O 2 NPs (x = 0.05, 0.10, 0.15 and 0.20) was synthesized by a similar way, only that lithium hydroxide (LiOH) of 0.05, 0.10, 0.15 and 0.20 by wt % was added in the mixture solution before adding NH 4 OH.

Electrodes fabrication for electrochemical properties study
The electrode slurries of Li x Ti 1-x O 2 NPs (x = 0, 0.05, 0.10, 0.15 and 0.20) were prepared by ball milling each product with PVDF and acetylene black at 80: 10: 10 wt% ratio in 500 µL NMP solvent at RT for 24 h.Each electrode was fabricated by dripping an active mass slurry of approximately 200 µL to coat on an area 1 cm 2 at one end of ultrasonically cleaned nickel foam sheet of size 1 × 2 cm 2 , and dried for 2 h at 80 °C.After that, all electrodes were pressed at 1.5 tons for 1 min, and immersed in 0.5 M Li 2 SO 4 aqueous electrolyte prior to electrochemical properties testing.The CV study was performed in an applied voltage of 0.0 to + 0.5 V at scan rate 10, 20, 30, 50, 100 and 200 mV s −1 .The GCD study was performed at applied current density 1.5, 2, 4, 6, 8, 10 and 15 A g −1 .The capacity retention was evaluated at the 2000th cycle of GCD test at 10 A g −1 .The GCD results were used for the calculation of specific capacitance (C s ) using Eq.(1) 24 , where I, Δt, m, and ΔV stand for the constant discharge current (A), discharge time (s), mass of active material in electrode (g) and potential window (V), respectively.
Additionally, the energy density (E sd ) and power density (P sd ) of electrodes were determined from the GCD results, using Eqs.( 2) and (3) 24 , respectively.

Characterizations
X-ray source with CuKα (λ = 1.5406Å) generated by X-ray diffractometer (Philips X'Pert) was used for crystal structure and phase identification of the products.Raman study using a laser of 532 nm excitation (DXR Smart, Thermo Scientific) was employed for TiO 2 phase verification.Moreover, in order to confirm the existence of various modes of vibration between Ti and O bonding in the TiO 2 crystalline structure, Fourier transform infrared spectroscopy (FTIR, Bruker, Senterra) was performed.The surface morphology inspection of products and particles size determination were accomplished by field emission scanning electron microscope (FE-SEM, FEI, Helios NanoLab G3 CX).In addition, the quantitative estimation for major elements in wt% of the products could be achieved using energy dispersive X-ray spectroscopy (EDS) with elemental mapping to display the distribution of elements.Furthermore, high magnified bright field images with selected area electron diffraction (SAED) patterns by transmission electron microscope (TEM, FEI, TECNAI G2 20) was performed for clearer observed products morphology and more accurate particles size determination, including phase and structure confirmation.An instrument of Autosorb1-Quantachrome was employed for the study of specific surface area and a type of pore distributed in samples through the Bruanauer Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) techniques, respectively.Finally, an equipment of Wuhan Corrtest Instruments Corp Ltd.(Model CS350 Potentiostat/Galvanostat) was used for electrochemical properties studies of all Li x Ti 1-x O 2 NPs electrodes to obtain the CV, GCD and EIS results.

Results and discussion
The XRD patterns with Rietveld refinement fitting of Li x Ti 1−x O 2 NPs are displayed in Fig. 1a-e.In Fig. 1a-e, the most dominant XRD peaks at 25.26°, 36.92°,47.96°, 53.92°, 55.01°, 62.83°, 70.24° and 75.04° correspond to the crystalline diffraction plane (101), (004), (200), (105), (211), (204), (116), and (215), respectively.The XRD results matched with the standard data of JCPDS: 21-1272 for the tetragonal anatase TiO 2 crystalline phase of space group: I41/amd 9,18,25 .However, in a sample of x = 0.15 and 0.20 (Fig. 1d, e), many peaks of monoclinic Li 4 Ti 5 O 12 phase with space group: C2/c observed at 17.32°, 30.95° and 44.49° correspond with the diffraction plane (111), (311) and (400), respectively, and matching to the standard data of JCPDS: 49-0207.The formation of Li 4 Ti 5 O 12 phase might be due to the direct interaction of excess Li with pure anatase crystalline TiO 2 phase during the growth process.It was suggested that this phase could provide a nonsymmetric stretching vibration of O-Ti-O that could result in reduced conductivity of the samples.Moreover, the cell parameter (a, b and c) with cell volume and various parameters (R wp , R p , R ex and GOF, definition for these parameters was given elsewhere) were evaluated by Rietveld refinement method using a standard data of JCPDS: 21-1272 (tetragonal phase with space group: I41/amd) and JCPDS: 49-0207 (monoclinic phase with space group: C2/c), as displayed in Fig. 1a-e, and summarize of the results was listed in Table 1.As seen in Table 1 and the excellent fitting of the XRD patterns in Fig. 1a-e, it can be concluded that Li loading significantly affect the cell parameters of anatase TiO 2 phase.Obviously, the cell parameters and cell volume of samples decrease with increasing Li loading, leading to the deceased crystallite size of Li x Ti 1−x O 2 NPs.Generally, Ti 4+ in a unit cell of TiO 2 crystal system is bonded to six equivalent O 2− atoms, leading to the formation of mixture distorted edge and corner-sharing TiO 6 octahedra.Furthermore, in a unit cell of Li 4 Ti 5 O 12 phase, a complicated structure is formed owing to the formation of LiO 4 tetrahedra by the bonding of Li 1+ with four O 2− atoms at the cell corners that could be shared with others two equivalent LiO 6 octahedra and ten TiO 6 octahedra.Moreover, the percentage of anatase TiO 2 phase and Li 4 Ti 5 O 12 phase were determined in samples of x = 0.15 and 0.20, and found to be (95.12 and 4.88%) and (92.23 and 7.77%), respectively.Additionally, the X-ray line of the diffraction planes (101), (004), (200), (105), (211), (204), (116), and (215) were used for the evaluation of average crystallite sizes (D Sh ) of all samples, using the Scherrer's equation (4).
In Eq. ( 4), the parameters θ, λ and β are defined for Bragg angle, wavelength of X-ray and full width at half maximum, respectively.k is the constant and was taken as 0.9.The evaluated D Sh values are 19.61 ± 0.59, 19.21 ± 0.56, 18.31 ± 0.52, 18.23 ± 0.64 and 18.12 ± 0.76 nm for Li x Ti 1-x O 2 NPs of x = 0, 0.05 0.10, 0.15 and 0.20, respectively.All the D Sh values were summarized in Table 1, and the plot of D Sh versus Li concentration is illustrated in Fig. 1f.Obviously seen in Fig. 1f, D Sh decreases with increasing Li concentration.The decreased D Sh was suggested to originate from the replacement of a large ionic radius of Ti 4+ (0.745 Å) and Ti 3+ (0.670 Å) by a small ionic radius of Li + (0.60 Å) in the anatase TiO 2 crystal structure.By comparing the ionic radius of (1) www.nature.com/scientificreports/Li + (0.60 Å) and Ti 4+ (0.745 Å), it is clear that a possible substitution of a small amount of Ti 4+ by Li + would be accompanied by a weak lattice expansion, due to the relatively small difference between their respective ionic radius, so that the Li + ions can be dissolve into anatase TiO 2 phase and Li 4 Ti 5 O 12 phase 26 .However, the substitution might induce lattice expansion, resulting in a shift of the anatase peak to the lower angles.Although for the replacement of Ti 4+ by L i+ ions, some Ti-O bonds are broken, which leads to the formation of oxygen vacancies, the contraction of lattice caused by oxygen deficiency is eliminated through lattice expansion induced by the presence of the slightly smaller lithium ions 11 .As a result, Li + ions appear to be an appropriate option for modifying the local crystal structure at Ti 4+ sites in TiO 2 , because they could operate as charge compensators and could additionally enhance capacitive properties due to the availability of more active sites 27 .FTIR spectra of samples are displayed in Fig. 2a-e.The broaden vibration peaks around 3250-3350 cm −1 are assigned for O-H stretching modes, relating to the stretching vibration of the hydroxyl (O-H) group due to the formation of H 2 O molecules on surface 11 .Moreover, the appeared vibration peaks around 1640-1644 cm −1 are designated to the symmetric stretching of Ti-OH on surface 18 .Additionally, the observed peaks around 1110-1113 cm −1 and 600-625 cm −1 indicated the bonding of Ti-O in an anatase TiO 2 structure 11 .In the samples with x = 0.10, 0.15 and 0.20, the observed peaks in a range 790-900 cm −1 are attributed to the symmetric C-H (a) (  www.nature.com/scientificreports/and asymmetric CH 2 vibrations of an organic polyethylene glycol that could not be completely removed after calcination 11 .The strong vibration peaks in a range 410-625 cm −1 are associated with the vibration modes of O-Ti-O bonding in an anatase TiO 2 structure.Further structural analysis of Li x Ti 1−x O 2 NPs (x = 0, 0.05, 0.10, 0.15 and 0.20) was performed by Raman technique, as shown in Fig. 3a-e.In these figures, the major sharp Raman shift observed at ~ 144 cm −1 corresponds to the E g(1) mode of anatase TiO 2 18,28 .The peak at 396 cm −1 corresponds to the B 1g(1) mode, while another at 639 cm −1 corresponds to the E g(2) mode, arising from the symmetric stretching mode of O-Ti-O bonding in crystallite anatase TiO 2 28 .The other one observed at 515 cm −1 was assigned for A 1g + B 1g(2) mode, corresponding to the antisymmetric bending vibration of O-Ti-O bonding in TiO 2 structure 18,28 .Therefore, Raman results confirmed the anatase phase of all samples.
Morphology and average particles size (Dps) of Li x Ti 1−x O 2 NPs are demonstrated by FE-SEM images with corresponding histograms in Fig. 4a-e.All images show the homogeneous distribution of agglomerated NPs with intercalated space between them, illustrating the micrographs of porous surface similar to spongy materials.However, a secondary monoclinic phase of Li 4 Ti 5 O 12 NPs that existed in the samples with x = 0.15 and 0.20 could not be observed or identified by SEM micrographs in Fig. d-1, d-2, e-1, e-2 was suggested to be owing to the small amount of them compared to a major TiO 2 phase, as estimated and listed in Table 1.The Dps values are 30.04± 4.92, 27.97 ± 6.56, 25.12 ± 2.64, 25.03 ± 5.53 and 24.66 ± 6.13 nm for samples of x = 0, 0.05 0.10, 0.15 and 0.20, respectively.The Dps values were listed in Table 1.As seen in Table 1, Dps decreases with increased Li loading.The uniform dispersion of agglomerated TiO 2 NPs in electrodes could lead to enhance the porosity and form the conducting networks for charge transfer, as suggested by Prashad et al. 20 .Moreover, the observed porous structure of Li-doped anatase TiO 2 NPs could increase the surface area of the electrode materials for the adsorption of electrolyte ions, resulting in more charges collection and could finally enhance the specific capacitance.Figure 5a-e display the EDS results and mapping of elements for Li x Ti 1−x O 2 NPs.The EDS results clearly show the uniform distribution of major elements Ti and O in the samples.However, Li element could not be detected due to its light-weight and a limitation of the instrument.The atomic percentages for Ti element were estimated to be about 55.0%, 53.7%, 52.9%, 50.9% and 50.5% for Li x Ti 1−x O 2 NPs with x = 0, 0.05 0.10, 0.15 and 0.20, respectively.The decreased amount of Ti was due to the Li replacement in anatase crystal structure of TiO 2 .
In fact, more accurate particles size of Li x Ti 1-x O 2 NPs can be evaluated from TEM bright field images, including a better clear morphology of particles, as illustrated in Fig. 6a-e.As obviously seen, all images display NPs of very fine cuboidal shape with irregular size and agglomerated to form a spongy like-structure with roughly estimated individual particle size in an interval of 5-10 nm.Notably, the particle sizes estimated by TEM are smaller than those evaluated by SEM, which might be due to the dispersion of NPs during the sonication process of samples preparation prior to TEM performance.Moreover, the median size of NPs could be slightly reduced with the inclusion of Li + ions on the Ti 4+ and Ti 3+ sites in the anatase TiO 2 crystalline structure 18,20 .Furthermore, the unique morphology and homogenous size distribution in a narrow range of TiO 2 NPs was suggested to result in the increase of materials porosity and surface area.Additionally, the halo ring shape with arranged spots on the circumferences of SAED patterns in all samples indicate a polycrystalline nature of the materials 18 .Moreover, all SAED patterns had been indexed to be composed of different crystalline planes that correspond to those of anatase phase TiO 2 , agreeing well with the XRD results shown in Fig. 1.NPs.Regarding to these results, the observed hysteresis loops of all samples exhibit the BET curve of type IV, corresponding to that of mesoporous materials 11,20 .The evaluated specific surface area of anatase TiO 2 NPs (x = 0) was 163.01 m 2 g −1 , whereas those of Li x Ti 1-x O 2 NPs (x = 0.05, 0.10, 0.15 and 0.20) displayed the larger values of 180.23, 246.94, 239.92 and 241.93 m 2 g −1 , respectively.Moreover, the average pore size and total specific pore volume of Li x Ti 1−x O 2 NPs (x = 0, 0.05, 0.10, 0.15 and 0.20) by the BJH technique were found to be 6.54 ± 0.93, 6.31 ± 0.76, 5.91 ± 0.66, 5.71 ± 0.58 and 5.80 ± 0.61 nm, and 0.30, 0.31, 0.35, 0.37 and 0.38 cm 3 g −1 , respectively.All of these values were listed in Table 1, and their plots as a function of Li concentration are illustrated in Fig. 7f.It was suggested that the great increase of specific surface area with small pore size of Li-doped anatase TiO 2 NPs could create the appropriate pathways for ions to diffuse into the surface of electrodes, resulting in the increased charge collection and improvement of the capacitive performance 11,20 .The morphology and pore size of the produced compounds indicated nanoscale particles that could provide high electrolyte-electrode interfacial surface area, resulting in the comprehensive permeation of electrolyte and minimizing the path of transport to accelerate the fast transfer of Li + and e − in Li-doped anatase TiO 2 NPs cathode 27 .Furthermore, the mesoporous nature of electrode could improve the access of electrolyte into the bulk of the materials, while also providing high power tapping densities and robust structural and electrical interconnectivity across the electrode 29 .www.nature.com/scientificreports/Cyclic voltammetry (CV) measurements of Li x Ti 1−x O 2 NPs (x = 0, 0.05, 0.10, 0.15 and 0.20) electrodes were performed at a scan rate 50 mV s −1 in 0.5 M Li 2 SO 4 electrolyte within a potential window 0.0-0.5 V.The capacitive performance of each electrode is shown in Fig. 8a.The electrochemical performance of all electrodes performed at different scan rates are displayed in Fig. 8b-f.As seen in Fig. 8a, the CV curve of Li 0.10 Ti 0.90 O 2 NPs electrode reveals the largest size, indicating a superior electrochemical performance compared to other electrodes.According to Fig. 8a-f, the distorted rectangular shape with apparent redox peaks of CV curves are observed, suggesting a typical pseudocapacitive behavior of Li x Ti 1−x O 2 NPs electrodes 4,16,20 .Moreover, all curves exhibit the stability with increasing scan rate through the whole applied voltage range 4,18,20 .Additionally, it is obviously seen that with enhanced potential sweep rate from 10 to 200 mV s −1 , the anodic and cathodic peaks are shifted to the negative and positive values, respectively.In addition, the appeared anodic and cathodic peaks in a voltage range 0.18-0.35V were due to the faradaic redox reaction of TiO 2 NPs.Generally, the appearance of redox peaks is correlated to the cation interaction on the TiO 2 surface, which can be expressed as 4,30   Owing to the excellent ability of alteration between different oxidation states of Ti ions during the redox reaction, TiO 2 was suggested to be a potential material for SCs electrode.Regarding to the redox reaction,  Ti 4+ was transferred to Ti 3+ while charging, and converted to the initial state during discharging 18,19 , showing a pseudocapacitive behavior of n-type semiconductor for all Li x Ti 1-x O 2 NPs electrodes.In addition, it was reported that during discharge, a number of Li + ions was inserted into the interstitial sites of the Li-doped TiO 2 NPs framework, which implied a partial reduction of Ti 4+ to Ti 3+ state 29,[31][32][33][34] .Moreover, it was also suggested that the redox reaction could be possibly affected by the thickness variation of diffusion layers in electrodes due to using different scan rates in the measurements.The obviously enhanced current with scan rates indicates a good rate capability of Li  www.nature.com/scientificreports/TiO 2 NPs electrode.The increased electrical conductivity and surface area of Li-doped anatase TiO 2 NPs with increasing Li content were suggested for the improved performance.Furthermore, the GCD results for electrochemical stability study at 10 A g −1 with 5000 cycles test are illustrated in Fig. 10c.According to these results, a high capacitance retention of 87.1%, 89.0%, 92.6%, 91.7% and 90.4% at the 5000th cycles were attained for Li x Ti 1−x O 2 NPs electrodes with x = 0, 0.05, 0.10, 0.15 and 0.20, respectively, and the values were listed in Table 2.
Obviously, all electrodes of Li-doped anatase TiO 2 NPs illustrated a superior capacitance retention as compared to undoped anatase TiO 2 NPs 18 .Actually, it has been observed that good stability after testing for 5000th cycles of all Li x Ti 1−x O 2 NPs electrodes in aqueous electrolyte of 0.5 M Li 2 SO 4 is the most interesting one, since three main effects are evident; (i) decrease in the aggregation and overlapping of Li x Ti 1−x O 2 NPs during a long time required by the charge-discharge process, (ii) the fast Li + ions diffusion and increased electronic conductivity on surface of Li x Ti 1−x O 2 NPs, and (iii) a slight shift of the CV and GCD curves is a very promising strategy to produce an environment friendly supercapacitor, which is able to reach in the future for the targeted energy and power density of organic electrolyte-based systems with acceptable good electrochemical performance [35][36][37][38] .
Additionally, the EIS results of all Li x Ti 1−x O 2 NPs electrodes obtained in a range 0.01-100 kHz frequency at  3. Therefore, in this study, we thoroughly explored the influence of Li-doped anatase-TiO 2 NPs on high-performance supercapacitors.Generally, pentavalent donor-type doping is required to increase the material's electrical conductivity, and Li + ions can easily substitute Ti 4+ and Ti 3+ ions in the anatase-TiO 2 lattice at a wide range of concentrations.According to the experimental results, adding a modest amount of Li to the material could potentially affect many factors such as crystal size, phase purity, morphology, surface area, and pore size distribution.Consequently,Li x Ti 1−x O 2 NPs electrode with x = 0.10 exhibited superior ion diffusivity, high conductivity, and small particle size compared to the other samples.As a result, Li 0.10 Ti 0.90 O 2 NPs electrode exhibited a higher electrochemical activity compared to the others electrode.The increased lattice parameters could improve its overall performance by enhancing its metallic-like character, while having minor effects on its electrochemical properties.The CV and GCD tests showed that the Li 0.10 Ti 0.90 O 2 NPs electrode exhibited a pseudocapacitive storing mechanism, which occurred on the electrode surfaces at Li-doped Ti sites.As a result, Li + ions played an important role in the improvement of electrical conductivity, charge storage capacity and stability with capacitance retention reaching as high as 92.6% after 5,000 cycles GCD test.

Conclusions
In summary, anatase Li x Ti 1−x O 2 NPs (x = 0, 0.05 0.10, 0.15 and 0.20) could be synthesized by the sol-gel process.The anatase phase with space group I41/amd of tetragonal Li x Ti 1-x O 2 NPs was confirmed by XRD results.Additionally, the monoclinic phase of Li 4 Ti 5 O 12 was detected in samples with x higher than 0.10, suggesting for the   1. www.nature.com/scientificreports/network of NPs for charges transfer.The BET results illustrated the increased surface area of Li x Ti 1−x O 2 NPs with increasing Li loading.Electrochemical studies showed the pseudocapacitive behavior of all samples with high-quality performance achieved in a sample of x = 0.10 that revealed the highest value of 822 F g −1 for specific capacitance at 1.5 A g −1 , and could retain 92.6% of its original value after 5000 cycles test.Therefore, Li 0.10 Ti 0.90 O 2 NPs with excellent performance in terms of high capacitance, high power density (176.04W kg −1 ) and high energy density (102.75W h kg −1 ) was suggested to be an appropriate material for supercapacitors electrodes application.

Figure 1 .
Figure 1.Rietveld refinement fitted XRD patterns of samples with different Li concentrations and plot of average crystallite size vs.Li concentration.

Figure 3 .
Figure 3. Raman results of Li x Ti 1−x O 2 NPs with different Li concentrations.

Figure 5 .
Figure 5. EDS results and mapping images of Li x Ti 1−x O 2 NPs.

Figure 7 .
Figure 7. (a-e) Nitrogen sorption isotherms results with inset showing average pore diameter of Li x Ti 1−x O 2 NPs.(f) Plots of BET surface area, average pore size and total specific pore volume (inset) as a function of Li concentration.

Figure 9 .
Figure 9. (a) GCD results of all Li x Ti 1−x O 2 NPs electrodes at 1.5 A g −1 .(b-f) GCD results of all Li x Ti 1−x O 2 NPs (x = 0.0-0.20)electrodes performed at different current densities.

Figure 10 .Table 2 .
Figure 10.Electrochemical performance of all electrodes, (a) specific capacitance vs. current density, (b) Ragone plots, (c) cycling stability at 10 A g −1 and (d) Nyquist plots of all electrodes in aqueous electrolyte of 0.5 M Li 2 SO 4 .

Wavenumber (cm -1 ) Li 0.20 Ti 0.80 O 2 NPs
FTIR results of Li x Ti 1−x O 2 NPs with different Li concentrations.